1. Field of the Invention
The present invention relates to a hydraulic damping force control unit, a hydraulic shock absorber, a front fork for a vehicle, and a hydraulic rotary damper which use a poppet valve to generate damping force.
2. Description of Related Art
One example of conventional hydraulic shock absorber having a damping force control valve is disclosed in Japanese Patent Document JP-B-3306526. The hydraulic shock absorber disclosed in JP-B-3306526 is for use in suspension systems of vehicles, and constructed with a hydraulic cylinder provided between the wheel side and the vehicle body frame side, damping force control valves provided in a hydraulic circuit of the hydraulic cylinder, and so forth.
The hydraulic cylinder is constructed with a cylinder body filled with hydraulic fluid, a piston which defines the inside of the cylinder body into a head side fluid chamber and a rod side fluid chamber, and so forth. The head side fluid chamber and the rod side fluid chamber are connected with each other through a hydraulic circuit including valves and an accumulator.
The hydraulic circuit includes a head side fluid path which connects the head side fluid chamber to the accumulator, a rod side fluid path which connects the rod side fluid chamber to the accumulator, a head side check valve provided on the head side fluid path to allow hydraulic fluid to flow toward the head side fluid chamber only, a rod side check valve provided on the rod side fluid path to allow hydraulic fluid to flow toward the rod side fluid chamber only, a compression side damping force control valve provided in parallel with the head side check valve, an expansion side damping force control valve provided in parallel with the rod side check valve, and so forth.
The compression side damping force control valve and the expansion side damping force control valve include a poppet valve, which is constructed such that its valve element to be described later is opened, when the check valve regulates the flow of hydraulic fluid to increase the hydraulic pressure, to generate damping force. The damping force control valves include a hydraulic fluid inlet connected to a fluid path located on the hydraulic cylinder side with respect to the check valve, a valve hole of which one end is located at the opening of the hydraulic fluid inlet, a hydraulic fluid outlet opening in the wall surface of the valve hole and connected to a fluid path located opposite the hydraulic cylinder with respect to the check valve, a valve element fitted and inserted in the valve hole so as to be reciprocally movable, a compression coil spring for urging the valve element in the closing direction (toward the hydraulic fluid inlet side), a pilot pressure chamber defined in the valve hole by the valve element, and so forth.
The valve element is formed in a cylinder having a tapered portion at its distal end. The tapered portion faces the hydraulic fluid inlet from inside the valve hole, and is formed to seat on a valve seat which is the opening edge of the hydraulic fluid inlet. When the damping force control valve is fully closed with the tapered portion seating on the valve seat, a portion of the tapered portion more centrally than the valve seat receives the hydraulic pressure at the hydraulic fluid inlet, and the outer circumference of the tapered portion located more internally of the valve hole than the valve seat receives the hydraulic pressure at the hydraulic fluid outlet.
The pilot pressure chamber is connected through a pilot path to a fluid path on the hydraulic cylinder side with respect to the check valve. At the middle of the pilot path is provided a relief valve which allows hydraulic fluid to flow to a fluid path located opposite the hydraulic cylinder with respect to the check valve when the hydraulic pressure in the pilot path has been increased to a predetermined value or higher.
According to the damping force control valve, when the hydraulic pressure in the fluid path located on the hydraulic cylinder side from the relief valve has exceeded the predetermined value to open the relief valve, the hydraulic pressure in the pilot pressure chamber is reduced, which results in the valve element being opened. As the valve element opens, hydraulic fluid passes through a space between the valve seat and the tapered portion to flow from the hydraulic fluid inlet into the valve hole, and then passes through the valve hole to be discharged through the hydraulic fluid outlet. At this time, the damping force control valve functions as a throttle to generate damping force. Hydraulic fluid discharged through the hydraulic fluid outlet flows toward the accumulator or the side of the other fluid chamber of the cylinder.
It should be noted that the applicant could not find any related art document closely related to the present invention by the time of filing the present patent application, except for the one specified in the related art document information section included herein.
When the hydraulic shock absorber is in an expansion stroke, for example, the rod side check valve is closed and thus hydraulic fluid passes through the expansion side damping force control valve and then through the head side check valve to flow into the head side fluid chamber.
Regarding the check valve, the elastic reaction force and the spring constant of its return spring are set to be higher, and the amount of valve lift is set to be smaller, in order to increase the responsiveness of the check valve when opened and closed. Therefore, a differential pressure is generated between the upstream side and the downstream side of the check valve when hydraulic fluid passes through it.
At this time, the pressure (differential pressure) which has occurred at the head side check valve is transmitted through the hydraulic fluid outlet of the compression side damping force control valve into the valve hole, which presses the outer circumference of the tapered portion in the opening direction to cause the valve element to be opened. In other words, when the hydraulic shock absorber is in the expansion stroke, the damping force control valve that should be on standby as closed is unfavorably opened.
When the frequency and amplitude of vibration of a wheel are relatively small, there is enough time before the expansion stroke of the hydraulic shock absorber ends. Thus, the flow rate of hydraulic fluid through the compression side damping force control valve and the differential pressure are gradually reduced, and along with that the hydraulic pressure is overcome by the elastic reaction force of the compression coil spring which is urging the valve element in the closing direction. This causes the valve element to be closed before the piston of the hydraulic shock absorber reaches dead center.
When the frequency and amplitude of vibration of a wheel are relatively large, however, the flow rate of pressurized fluid through the head side check valve is high and hence the differential pressure is large. This causes the compression side damping force control valve to be opened to a relatively large degree. Thus, occasionally, there is not enough time for the valve element to be closed before the piston of the hydraulic shock absorber reaches dead center in the expansion stroke and the valve is kept opened even after the hydraulic shock absorber has shifted to a compression stroke.
If the compression side damping force control valve is opened when the hydraulic shock absorber has shifted from the expansion stroke to the compression stroke as described above, the compression side damping force control valve cannot generate a magnitude of damping force necessary at the moment. When the hydraulic shock absorber has shifted to the compression stroke, the hydraulic pressure on the upstream side of the compression side damping force control valve (or the hydraulic pressure in the hydraulic fluid inlet and in the pilot pressure chamber) is gradually increased. The compression side damping force control valve is abruptly closed from its opened state by the elastic damping force of the compression coil spring and the hydraulic pressure in the pilot pressure chamber, and then the valve element is gradually opened to generate damping force.
In other words, the damping force to be generated by the compression side damping force control valve at this time is unstable (the damping force waveform is disordered. In order for the damping force to be increased according to the increase in the speed of the hydraulic shock absorber, the damping force control valve needs to be closed when the piston starts to move. The above description is based on the case where the hydraulic shock absorber shifts from the expansion stroke to the compression stroke. On the contrary, when the hydraulic shock absorber shifts from the compression stroke to the expansion stroke, the expansion side damping force control valve is on standby as opened, which causes the same problem as described above.
The damping force of such a conventional hydraulic shock absorber changes as shown for example in FIG. 11. FIG. 11 is a graph showing the relationship between the moving speed of a piston and the damping force. As shown in FIG. 11, because the compression side damping force control valve, which should be closed, is opened when the hydraulic shock absorber has shifted from the expansion stroke to the compression stroke, the damping force is unstable in the range indicated by symbol a in FIG. 11. Also, because the expansion side damping force control valve, which should be closed, is opened when the hydraulic shock absorber has shifted from the compression stroke to the expansion stroke, the damping force is unstable in the range indicated by symbol b in FIG. 11.